Toxoplasma gondii is an important opportunistic pathogen in patients with acquired and primary deficiencies in T cell functions. The overall aim of this proposal is to understand the factors that regulate the development and maintenance of T cell responses required for resistance to T. gondii. The early events that occur during infection have a profound influence on the development of NK and T cell production of IFN-gamma required for resistance to this pathogen. While there are many factors such as cytokines and the presentation of parasite antigen to specific T cells, that regulate the activation of these protective responses, the second signals provided by co-stimulation also play a role in innate and adaptive immunity to T. gondii. In order to fully understand how protective immunity to toxoplasmosis is initiated and maintained we need to understand which co-stimulatory pathways are important and the mechanisms that underlie their effects. Our previous studies have demonstrated a critical role for CD28 in the generation of CD4 v T cell responses required for protective memory responses and the proposed studies will utilize in vivo studies with transgenic mice to understand the mechanisms whereby CD28 mediated production of T cell growth factors or anti-apoptotic molecules underlies this effect. In addition, our studies have shown that there are CD28-independent mechanisms that regulate NK and T cell responses to T. gondii and we have identified the Co-stimulatory molecule ICOS as being important in CD28-independent resistance to T. gondii. The proposed studies will integrate molecular and cellular approaches to address the relationship between ICOS and CD28 and their role in innate and adaptive responses to T. gondii and determine whether agonists of ICOS and CD28 can be used therapeutically to prevent disease caused by this opportunistic pathogen.